The present invention relates to a method and an apparatus for applying a coating liquid to a moving web or film, and particularly relates to a method of coating a high speed moving web with a thin layer of photosensitive material.
Conventionally, photosensitive materials have been applied to moving web according to a method wherein a backing roller in contact with the web is impressed with a DC voltage. Such discharge processing methods have the advantage of increasing the adhesive force between the web and the coating layer, and also reduce the problem of nonuniform liquid application due to air entrapment between the web and the coating liquid. Such a method is disclosed in British Pat. No. 1,130,109. In other methods, a direct current is applied during pre-processing steps at the liquid application section. Such arrangements are disclosed in Japanese unexamined patent application publications Nos. 55-142565 (1980), 61-146369 (1986) and 61-14670 (1986).
However, the aforementioned DC voltage application methods have serious disadvantages. In these methods, air entrapment between the web and the coating liquid caused by the enfolding of air between the web and the coating liquid is suppressed by strengthening the adhesion between the coating liquid and the web by adding an electrostatic attraction force in addition to the ordinary intermolecular force acting therebetween. However, it is rather difficult to apply a uniform electrostatic attractive force over the entire width of the web when the width dimension of the web is large. In such cases, the nonuniformity of the applied electrostatic force causes the appearance of streaks in the finished web after it has been coated with the photosensitive material. Such streak defects render the resultant product unusable. Especially with web widths ranging up to 1 meter or more, this problem is particularly significant.
In methods where the web surface is charged by means of corona discharging preceding the coating process, the discharge process cannot be uniformly performed. Particularly, an unevenness in the charge distribution is generated because it it not possible to maintain a uniform distance between the electrode and the opposing surface of the web because of the roughness of the web surface. There may also be some unevenness in the chemical composition of the web, leading to the same result.
In methods wherein a DC voltage is applied via a backing roller, unevenness in the charge distribution at the surface of the web is caused due to variations in the thickness of the web and the uneven dielectric distribution in the web.